Microparticles, microspheres, and microcapsules are solid or semi-solid particles having a diameter of less than one millimeter, more preferably less than 100 microns, which can be formed of a variety of materials, including synthetic polymers, proteins, and polysaccharides. Microspheres have been used in many different applications, primarily separations, diagnostics, and drug delivery.
A number of different techniques can be used to make these microspheres from synthetic polymers, natural polymers, proteins and polysaccharides, including phase separation, solvent evaporation, emulsification, and spray drying. Generally, the polymers form the supporting structure of these microspheres, and the drug of interest is incorporated into the polymer structure. Exemplary polymers used for the formation of microspheres include homopolymers and copolymers of lactic acid and glycolic acid (PLGA) as described in U.S. Pat. No. 5,213,812 to Ruiz, U.S. Pat. No. 5,417,986 to Reid et al., U.S. Pat. No. 4,530,840 to Tice et al., U.S. Pat. No. 4,897,268 to Tice et al., U.S. Pat. No. 5,075,109 to Tice et al., U.S. Pat. No. 5,102,872 to Singh et al., U.S. Pat. No. 5,384,133 to Boyes et al., U.S. Pat. No. 5,360,610 to Tice et al., and European Patent Application Publication Number 248,531 to Southern Research Institute; block copolymers such as tetronic 908 and poloxamer 407 as described in U.S. Pat. No. 4,904,479 to Illum; and polyphosphazenes as described in U.S. Pat. No. 5,149,543 to Cohen et al. Microspheres produced using polymers such as these exhibit a poor loading efficiency and are often only able to incorporate a small percentage of the drug of interest into the polymer structure. Therefore, substantial quantities of microspheres often must be administered to achieve a therapeutic effect.
Spherical beads or particles have been commercially available as a tool for biochemists for many years. For example, antibodies conjugated to beads create relatively large particles specific for particular ligands. The large antibody-coated particles are routinely used to crosslink receptors on the surface of a cell for cellular activation, are bound to a solid phase for immunoaffinity purification, and may be used to deliver a therapeutic agent that is slowly released over time, using tissue or tumor-specific antibodies conjugated to the particles to target the agent to the desired site.
A common method of covalently binding an antibody to a solid phase matrix is to derivatize a bead with a chemical conjugating agent and then bind the antibody to the activated bead. The use of a synthetic polymeric bead rather than a protein molecule allows the use of much harsher derivatization conditions than many proteins can sustain, is relatively inexpensive, and often yields a linkage that is stable to a wide range of denaturing conditions. A number of derivatized beads are commercially available, all with various constituents and sizes. Beads formed from synthetic polymers such as polyacrylamide, polyacrylate, polystyrene, or latex are commercially available from numerous sources such as Bio-Rad Laboratories (Richmond, Calif.) and LKB Produkter (Stockholm, Sweden). Beads formed from natural macromolecules and particles such as agarose, crosslinked agarose, globulin, deoxyribose nucleic acid, and liposomes are commercially available from sources such as Bio-Rad Laboratories, Pharmacia (Piscataway, N.J.), and IBF (France). Beads formed from copolymers of polyacrylamide and agarose are commercially available from sources such as IBF and Pharmacia. Magnetic beads are commercially available from sources such as Dynal Inc. (Great Neck, N.Y.).
One disadvantage of the microparticles or beads currently available is that they are difficult and expensive to produce. Microparticles produced by these known methods have a wide particle size distribution, often lack uniformity, and fail to exhibit long term release kinetics when the concentration of active ingredients is high. Furthermore, the polymers used in these known methods are dissolved in organic solvents in order to form the microparticles. They must therefore be produced in special facilities designed to handle organic solvents. These organic solvents could denature proteins or peptides contained in the microparticles. Residual organic solvents could be toxic when administered to humans or animals.
In addition, the available microparticles are rarely of a size sufficiently small to fit through the aperture of the size of needle commonly used to administer therapeutics or to be useful for administration by inhalation. For example, microparticles prepared using polylactic glycolic acid (PLGA) are large and have a tendency to aggregate. A size selection step, resulting in product loss, is necessary to remove particles too large for injection. PLGA particles that are of a suitable size for injection must be administered through a large gauge needle to accommodate the large particle size, often causing discomfort for the patient.
Generally, many currently available microparticles are activated to release their contents in aqueous media and therefore must be lyophilized to prevent premature release. In addition, particles such as those prepared using the PLGA system exhibit release kinetics based on both erosion and diffusion. In this type of system, an initial burst or rapid release of drug is observed. This burst effect can result in unwanted side effects in patients to whom the particles have been administered.
Microparticles prepared using lipids to encapsulate target drugs are known. For example, lipids arranged in bilayer membranes surrounding multiple aqueous compartments to form particles may be used to encapsulate water soluble drugs for subsequent delivery, as described in U.S. Pat. No. 5,422,120 to Sinil Kim. These particles are generally greater than 10 microns in size and are designed for intra-articular, intrathecal, subcutaneous and epidural administration. Alternatively, liposomes have been used for intravenous delivery of small molecules. Liposomes are spherical particles composed of a single or multiple phospholipid and cholesterol bilayers. Liposomes are 30 microns or greater in size and may carry a variety of water-soluble or lipid-soluble drugs. Liposome technology has been hindered by problems including purity of lipid components, possible toxicity, vesicle heterogeneity and stability, excessive uptake and manufacturing or shelf-life difficulties.
An objective for the medical community is the delivery of nucleic acids to the cells in a subject, including but not limited to an animal or a mammal, for treatment. For example, nucleic acids can be delivered to cells in culture (in vitro) relatively efficiently, but nucleases result in a high rate of nucleic acid degradation when nucleic acid is delivered to animals (in vivo).
In addition to protecting nucleic acid from nuclease digestion, a nucleic acid delivery vehicle must exhibit low toxicity, must be efficiently taken up by cells and have a well-defined, readily manufactured formulation. As shown in clinical trials, viral vectors for delivery can result in a severely adverse, even fatal, immune response in vivo. In addition, this method has the potential to have mutagenic effects in vivo. Delivery by enclosing nucleic acid in lipid complexes of different formulations (such as liposomes or cationic lipid complexes) has been generally ineffective in vivo and can have toxic effects. Complexes of nucleic acids with various polymers or with peptides have shown inconsistent results and the toxicity of these formulations has not yet been resolved. Nucleic acids have also been encapsulated in polymer matrices for delivery but in these cases the particles have a wide size range and the effectiveness for therapeutic applications has not been demonstrated.
Therefore, there is a need for addressing nucleic acid delivery issues, and providing effective nucleic acid formulations. Also, there is an ongoing need for development of microspheres and to new methods for making microspheres. Microspheres and their preparation have been described in U.S. Pat. No. 6,458,387 to Scott et al., U.S. Pat. No. 6,268,053, U.S. Pat. No. 6,090,925, U.S. Pat. No. 5,981,719 and U.S. Pat. No. 5,599,719 to Woiszwillo et al., and U.S. Pat. No. 5,578,709 to Woiszwillo. The foregoing references and all other references identified herein are incorporated herein by reference.